β-amylase is a starch-degrading enzyme which hydrolyzes alpha-1,4 bonds. It is found, e.g., in bacteria and plants and it breaks down starch mainly into maltose at the non-reducing end of the starch chain. β-amylase is abundant, e.g., in grains, where it converts the nutrient store of the cereal, i.e., starch, into sugar, if necessary. In cereals starch is mainly stored in the form of amylose and amylopectin. β-amylase converts all of the amylose into maltose, whereas about 60% of the amylopectin is converted into maltose and the rest into dextrin.
β-amylase is a commercially significant enzyme which is used, e.g., in the starch industry to produce maltose. Products containing large amounts of maltose are used e.g. in the confectionery and food industry. β-amylase has been isolated both from bacteria and from plants. For example, it has been obtained from Bacillus bacteria (U.S. Pat. No. 4,970,158 and JP Patent No. 60,126,080) and from thermostable Clostridium bacteria (U.S. Pat. No. 4,647,538). In addition to maltose, β-amylases derived from bacteria produce considerable amounts of maltotriose, whereas plant-based β-amylases produce relatively more maltose and thus they are more suitable for processes where the purpose is to obtain as sweet and/or fermentable products as possible. Besides, large-scale production of β-amylase from bacteria is difficult. The β-amylase used in industry is plant-based, in which case usually cereal, particularly barley or wheat, but also soybeans are used as the enzyme source.
During the growth, β-amylase is formed in the grains, where it is stored. A grain consists of a germ and a starch-containing endosperm, i.e. a kernel, which are separated from each other by a scutellum. The endosperm is surrounded by an aleutrone layer and the whole grain is surrounded by a pericarp layer, a testa layer and the actual husk. Wheat has no proper husk, but the pericarp and testa form a hard outer shell. β-amylase accumulates mainly in the endosperm and scutellum. The largest amounts of β-amylase are found in the outermost portions of the endosperm immediately underneath the aleurone layer.
β-amylase of barley has been studied thoroughly. This β-amylase and its production are described, e.g., in the following publications: D. E. Briggs, Barley, Chapman & Hall, London, 1978; Cook, Barley and Malt, Academic Press, London, 1962; J. R. A. Pollock, Brewing Science, Academic Press, London, 1979. The systematic name of the enzyme is 1,4-alpha-D-glucan maltohydrolase (Enzyme Classification (“EC”) 3.2.1.2). In the past, the β-amylase of cereal was separated first by grinding or milling the grain and then by extracting the β-amylase with water or a buffer. Purification of enzyme from extract of this kind is naturally difficult and laborious because, in addition to the enzyme concerned, the extract contains several other soluble components of the grain. Attempts have been made to improve separation of β-amylase from a solution containing it, e.g., by adsorbing the enzyme with polymer in the presence of ammonium sulphate (U.S. Pat. No. 5,294,341). Release of β-amylase from gluten has been experimented with protease (JP Patent No. 63,079,590).
β-amylase has also been isolated from the waste liquid of wheat starch production by adding sodium alginate and by recovering the coagulated enzyme (JP Patent No. 60,027,383) or by forming a calcium phosphate gel to which the enzyme adsorbs and from which it is then recovered (JP Patent No. 63,248,389). Waste liquid from starch production is not a good source of β-amylase because it is very dilute and contains large amounts of other components, which makes purification and concentration difficult and, as a result, the yield is low.
To obtain a purer raw extract and to avoid difficult downstream processing, it has been suggested that β-amylase be extracted from whole or partly husked grains. When e.g. barley grains are husked in such a manner that their endosperm does not break, the outmost layers of the endosperm function as a kind of filter which prevents access of insoluble substances to the steep water and restricts the access of soluble substances. It is preferable to carry out extraction in the presence of a reducing substance which releases β-amylase from other proteins of the grain (FI Patent No. 61,516 and U.S. Pat. No. 4,675,296).
A process for extracting β-amylase from cereal which reduces the cereal extraction time and improves the yield of enzyme has now been invented. The process is simple to carry out and is particularly suitable for processing husked cereal, which also facilities further purification of the enzyme.